The accurate measurement of the composition of gas or liquid chemical streams by an in-situe spectrographic or spectro scopic system presents a number of challenges. By example, for a spectrographic system to be used aboard a vehicle the spectrographic system must be rugged and capable of operating in a hostile environment that is subject to wide fluctuations in temperature, and which furthermore may be periodically subject to extreme vibration. The spectrographic system should also preferably be of small size and low cost, while providing accurate and repeatable results; should include a minimum number of moving parts; should operate in a rapid, real-time manner, and should be simple to install, maintain and operate.
Three conventional approaches to measuring the composition of a stream of gas or liquid can be generally categorized as Fourier Transform Infrared (FTIR) Spectroscopy, Non-Dispersive Infrared (NDIR) Spectroscopy, and Diode Array or Scanned Dispersive Spectroscopy (SDS).
The FTIR system is a general purpose system that is capable of quantifying a variety of gases or liquids. However, the FTIR system typically includes moving optical components which may not be capable of providing real-time (i.e., less than one second) information regarding the molecular species concentration within a sample stream. This is due primarily to the time required to obtain an interferogram, and the time that is then required to convert the interferogram to molecular species concentrations. In general, the FTIR system is bulky, costly, and temperature-sensitive, making its use as an in-situe gas or liquid analyzer less than optimum.
The NDIR measurement system has been employed for sampling the exhaust gas plume from stationary vehicles by comparing the absorption of radiation along two different optical paths. A first, or reference path contains, by example, air; while a second, sample path may include a gas cell containing a specific gas, such as CO or CO.sub.2, that is intended to be measured. Alternate approaches may utilize filters instead of gas cells. In either approach the conventional NDIR system, due at least to the requirement to provide two separate optical paths, is also bulky, costly, and may be temperature-sensitive, making its use as an in-situe emission analyzer less than optimum.
The SDS system utilizes an optical element, such as a grating, in combination with a mechanism for measuring and sequentially scanning a dispersed spectrum. Conventional systems are known to include a photodiode array used with parallel to serial multiplexing, or a mechanically translated photomultiplier. In general the SDS system is sensitive to vibration, and requires an accurate alignment of the integration and readout elements with the optical elements that disperse the spectrum.
Commonly assigned U.S. patent application Ser. No. 08/119,788, filed Sep. 10, 1993, (abandoned in favor of Ser. No. 08/318,566, filed Oct. 5, 1994), entitled "Optical Sensing Apparatus For Remotely Measuring Exhaust Gas Composition of Moving Motor Vehicles" by Michael D. Jack et al. teaches an IR-based system that measures the relative concentrations of several pollutants in the exhaust plume of a moving or a stationary vehicle. This system employs a number of adjacently spaced photodetectors that are sensitive to different wavelengths corresponding to spectral absorption peaks of constituents of the composition of the exhaust plume, including carbon monoxide, carbon dioxide, and hydrocarbons. This system is generally intended for roadside use, and not as an on-board component of a vehicle.